Thin film transistors (TFTs) are a type of field effect transistors (hereinafter referred to as FETs). TFTs are three-terminal elements having a gate terminal, a source terminal, and a drain terminal in the basic structure. TFTs are active elements having a function of switching the current between the source terminal and the drain terminal so that a semiconductor thin film deposited on a substrate is used as a channel layer in which electrons or holes move and a voltage is applied to the gate terminal to control the current flowing in the channel layer. TFTs are electronic devices that are most widely used these days in practical application. Typical applications of TFTs include liquid-crystal driving elements.
Currently, most widely used TFTs are metal-insulator-semiconductor-FETs (MIS-FETs) in which a polycrystalline silicon film or an amorphous silicon film is used as a channel layer material. MIS-FETs including silicon are opaque to visible light and thus fail to form transparent circuits. Therefore, when MIS-FETs are used as switching elements for driving liquid crystals in liquid crystal displays, the aperture ratio of a display pixel in the devices is small.
Due to the recent need for high-resolution liquid crystals, switching elements for driving liquid crystals now require high-speed driving. In order to achieve high-speed driving, a semiconductor thin film in which the mobility of electrons or holes is higher than that in at least amorphous silicon needs to be used as a channel layer.
Under such circumstances, Patent Document 1 proposes a transparent semi-insulating amorphous oxide thin film which is a transparent amorphous oxide thin film deposited by vapor deposition and containing elements of In, Ga, Zn, and O. The composition of the oxide is InGaO3(ZnO)m (m is a natural number less than 6) when the oxide is crystallized. The transparent semi-insulating amorphous oxide thin film is a semi-insulating thin film having a carrier mobility (also referred to as carrier electron mobility) of more than 1 cm2 V−1 sec−1 and a carrier density (also referred to as carrier electron density) of 1016 cm−3 or less without doping with an impurity ion. Patent Document 1 also proposes a thin film transistor in which the transparent semi-insulating amorphous oxide thin film is used as a channel layer.
However, as proposed in Patent Document 1, the transparent amorphous oxide thin film (a-IGZO film) containing elements of In, Ga, Zn, and O and deposited by any method of vapor deposition method selected from sputtering and pulsed laser deposition method has an electron carrier mobility in the range of only about from 1 to 10 cm2 V−1 sec−1, and thus it is pointed out that this carrier mobility is insufficient when the transparent amorphous oxide thin film is formed into a channel layer in TFTs.
In addition, in Patent Document 2, a sputtering target containing a sintered body which contains In, Ga, and Mg and one or more kinds of compounds selected from a compound represented by In2O3, a compound represented by In(GaMg)O4, a compound represented by MgGa2O4, and a compound represented by In2MgO4 has been proposed.
However, the target disclosed in Patent Document 2 includes a phase such as Ga2MgO4, which has low conductivity so as to cause arcing, and thus there is a problem that abnormal discharge is caused.
Therefore, it is the present situation that the development of an oxide sintered body or target for an oxide conductive film which does not include these phases that cause arcing is difficult.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-219538    Patent Document 2: PCT International Publication No. WO2013/005400    Patent Document 3: PCT International Publication No. WO2003/014409    Non-Patent Document 1: N. Ueda and six others, “New oxide phase with wide band gap and high electroconductivity, MgIn2O4”. Appl. Phys. Lett. 61 (16), 19, Oct. 1992, p. 1954-1955    Non-Patent Document 2: M. Orita and three others, “New Transparent Conductive Oxides with YbFe2O4 Structure”, JJAP, 34, L1550    Non-Patent Document 3: A. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano, and H. Hosono, Thin Solid Films 486, 38 (2005)